Enhancement: Support printing snapshot size in `snapshots` command

The `snapshots` command now supports printing the snapshot size for snapshots
created using this or a future restic version. For this, the `backup` command
now stores the backup summary statistics in the snapshot.

The text output of the `snapshots` command only shows the snapshot size. The
other statistics are only included in the JSON output. To inspect these
statistics use `restic snapshots --json` or `restic cat snapshot <snapshotID>`.

https://github.com/restic/restic/issues/693
https://github.com/restic/restic/pull/4705
